Injection molding machines are used to produce plastic articles for use in a variety of applications. In general, plastic in liquid form is injected into a cavity which is formed by two mold assemblies. Once the plastic has cooled, the two mold assemblies are separated so as to remove the formed article. Upon separation, the molded article remains in one of the mold assemblies. In order to effect the removal of the formed article from the mold plate, some means of ejecting the article is typically required. A variety of approaches exist for effecting ejection, including the use of ejector or stripper plates, stripping pins and pressurized air.
Ejector plates are typically associated with one of the mold assemblies and act against the formed article, to force the article from the mold plate. Normally, force is applied to the ejector plate at a single location, which may be located at or near the center of the ejector plate. The ejector plate acts upon ejector pins which in turn force the molded article out of the mold assembly. These systems are called single knockout ejector systems. In certain applications, single knockout ejector systems are very effective. However, in other instances, the plastic article can be of a very complicated design. For example, an article may include deep part walls, deep part ribs, cored holes, and other non-uniform attributes. For these more complicated designs, single knockout ejector systems are less effective. The non-uniform attributes result in unequal forces being generated as the formed article is being ejected from the mold plate. Accordingly, misalignment of the ejector plate within the mold assembly may occur, resulting in cock of the ejector plate. If this situation is not corrected, the ejector plate will eventually bind, forcing the injection molding machine to be shut down and serviced, as well as causing damage to the mold assembly which must be repaired or replaced.
One approach to solving this problem, has been to incorporate rack and pinion gears into mold assemblies for articles which have non-uniform attributes. While this approach is effective, it is also very expensive. For example, the mold assemblies are constructed uniquely for a given article to be molded. In order to allow for manufacture of replacement parts, the mold assemblies are stored when not in use. Thus, re-use of parts within the mold assemblies for other mold assemblies is not a normal practice. Accordingly, each such mold assembly must incorporate a dedicated set of rack or pinion gears.
Therefore, it is desirable to provide an ejection mechanism that compensates for the effects of unequal forces as an article is being ejected. It is also desirable that the ejection mechanism be easily adapted for use with a variety of article shapes without the need to add manufacturing steps to the manufacture of mold assemblies. It is further desirable that the ejection mechanism be more cost effective than prior art approaches which use dedicated rack and pinion gears for each mold assembly. Moreover, it is desired that the ejection mechanism use common and relatively inexpensive components.